1. Field of the Invention
The present invention relates to an image compression technique for applying compression processing to an original image.
2. Description of the Related Art
The digitizing of documents is proceeding owing to the growing popularity of scanners in recent years. When it is attempted to store a digitized document in a full-color bitmap format, a very large amount of memory is required. For example, in the case of size A4, the amount of data is approximately 24 MB at 300 dpi. Such a large amount of data is not a size suitable for transmission by being attached to e-mail or the like. Accordingly, the usual practice is to compress full-color images. JPEG compression is an example of a compression scheme widely used. Further, the specification of Japanese Patent Laid-Open No. 2002-077633 describes an image processing apparatus that employs a compression scheme according to which a text region is extracted from an original image, a text image corresponding to the text region and an image from which the text region is excluded are generated and each of these is subjected to different compression processing.
However, when an attempt is made to raise the compression rate with ordinary JPEG compression, so-called mosquito noise increases in the text portions, which are the important portions of the document, and readability declines as a result. Further, with the method described in Japanese Patent Laid-Open No. 2002-077633, good image quality is not obtained with regard to text portions unless image resolution is greater than a certain value. This will be described further with reference to FIGS. 2A to 2E. FIGS. 2A and 2D illustrate images in a case where images obtained by scanning the same document at 100 dpi and 300 dpi, respectively, are displayed at an equivalent pixel magnification. Images obtained by binarizing these are as shown in FIGS. 2B and 2E, respectively. For the purpose of comparison, FIG. 2C illustrates the result of simply enlarging a 100-dpi binarized image to a size the same as that of a 300-dpi image. It will be appreciated from a comparison of FIGS. 2C and 2E that when a low-resolution image is binarized, text quality declines to an extreme degree. Further, it is very difficult to apply a correction in such a manner that a low-resolution binarized image will come to have improved readability. Accordingly, in case of a low-resolution image, it is difficult to improve text readability with the arrangement described in Japanese Patent Laid-Open No. 2002-077633, which generates a text image using a binarized image obtained by binarizing an original image. In addition, even if an image is a high-resolution image, a similar problem arises with regard to very small characters.
On the other hand, if resolution is raised before an original image is compressed and then this higher-resolution image is compressed as an original image using the method of Japanese Patent Laid-Open No. 2002-077633, the image quality of text portions will be improved. However, since the original image has a high resolution, the compression rate declines (the file size becomes too large). Further, since the number of times image conversion is performed in order to compress the background image portion increases by one, this causes a decline in image quality.